A Phase II study of temozolomide (TMZ), combined with the AGT inhibitor, O6-benzylguanine (O6-BG), will be performed in pediatric patients with progressive or recurrent high-grade gliomas or brainstem gliomas to determine the activity of this drug combination in this patient population. O6-BG will be administered as a 60-minute intravenous (IV) infusion daily for 5 days of a 28-day course. TMZ capsules will be administered orally 30 minutes after completion of each O6-BG infusion.The primary objective is to determine the objective response rate to the combination of intravenous O6-BG and oral TMZ in children with progressive or recurrent high-grade gliomas or brainstem gliomas.